The use of infrared gas analyzers is becoming of increasing importance in several different fields. For example, in the medical field, infrared gas analyzers may be used to monitor concentration of gases in the blood or in a sample of expired air. In conjunction with pollution control, infrared gas analyzers may be used to monitor air pollutants deleterious to the environment.
A particular method for monitoring CO.sub.2 in the blood is by means of a transcutaneous measurement at the surface of the skin. Typically, the outermost layer of a small area of skin on a patient is removed, e.g., by repeated application of a strip of adhesive tape. A small pill box type device may then be sealed onto the skin so that the atmosphere in the box can come into pressure equilibrium with the body fluids in the skin. The concentration of CO.sub.2 in the pill box may then be measured by a suitable infrared gas analyzer which is preferably included within the pill box.
An effective transcutaneous CO.sub.2 measurement of the kind described above requires that the infrared gas analyzer be capable of providing a very rapid response time while utilizing only a small sample volume. Furthermore, the detector output should be insensitive to extraneous variables such as the intensity of any light sources employed, detector bias, and aging effects. Additionally, in medical applications of this kind the detector will be exposed to various sterilizing agents as well as contaminants present in the body fluids. The detector output should therefore be insensitive to contamination from these sources.